the amplitude, duration, and frequency of encoded information of receptors is received

by the nervous system. The prosthesis cannot respond to external stimuli due to the gap

between readable devices and neutrally intelligible. Signal coding and transmission are

converted into an electrical response by the sensor that can communicate with the ner­

vous system. In recent years, many attempts have been taken for establishing the bridge

between electrical devices and the systems of the human body.

9.3.2.1 Analog Signal Conversion and Amplification

The prosthetic device converts the output of the sensor into current and voltage. It is

the first step for the coding of information to interface with the central nervous system.

In most prosthetic devices, external stimuli like heat or pressure are not converted

directly into current and voltage. But these devices need an additional circuit that

converts the capacitive readout into voltage for example in capacitive pressure sen­

sors [30]. Pressure changes can be responded to by changing the transistor drain cur­

rent; either by integrating the transistor gate with piezocapacitive/piezoelectric senor

or transistor source with a piezoresistive sensor. The transistor in these devices works

like an amplifier that amplifies the readout signal for better sensitivity. Low gate ca­

pacitance and mobility limits these sensors to work at higher voltage, which requires

safety concerns [31].

9.3.2.2 Biomimetic Analog to Digital Transform

Action potential, in the form of a digital signal, is transmitted in subcutaneous receptors.

After the conversion of signal into current and voltage, is digitized into biological signals.

Digitizing of signals is performed by transistors. Silicon-based transistors are the tradi­

tional form that is made up of elastomers. Mechanical modulus of these transistors

mismatch with the human e-skin. Although scientists have explored the ways to integrate

these transistors with flexible systems by converting these into wires for connection with

a rigid surface. Organic transistors have more flexibility and are used in electronic skin

systems. The organic transistors convert pressure signals into frequency. The ring oscil­

lator in these systems is made up of plastic polyethylene naphthalte foils printed with

organic transistors, which converts the voltage input into a periodic voltage signal. Each

ring oscillator is equipped with a pair of complementary transistors. Resistance of the

pressure sensor is measured with a piezoresistive sensor that determines its output fre­

quency which later turned into pulse signals.

9.3.2.3 Synaptic Signal Processing

Synapses transfer the signal from one neuron into another neuron in the nervous system.

The signal is not directly transmitted but undergoes learning and memory, i.e., time and

intensity modulation occurs known as synaptic plasticity. Signal processing can bring the

learning and recovery of specific functions for the disabled in electronic skin-based

prosthetics. Synaptic plasticity can be well performed with ion-gated transistors. The ion-

gated transistor integrated with a tensile sensing element provides synaptic plasticity in

response to pressure as stimuli. Channel current of the ion-gated transistor encodes the

information on the frequency, amplitude, and duration of the applied stimulus. Synaptic

transistors can regulate the triboelectric and piezoelectric signals, while memristor-based

systems simulate the synaptic plasticity.

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Bioelectronics